FLINK基础（140）：DS流与表转换(6) Handling of Changelog Streams(1)简介

Internally, Flink’s table runtime is a changelog processor. The concepts page describes how dynamic tables and streams relate to each other.

A StreamTableEnvironment offers the following methods to expose these change data capture (CDC) functionalities:

• fromChangelogStream(DataStream): Interprets a stream of changelog entries as a table. The stream record type must be org.apache.flink.types.Row since its RowKind flag is evaluated during runtime. Event-time and watermarks are not propagated by default. This method expects a changelog containing all kinds of changes (enumerated in org.apache.flink.types.RowKind) as the default ChangelogMode.

• fromChangelogStream(DataStream, Schema): Allows to define a schema for the DataStream similar to fromDataStream(DataStream, Schema). Otherwise the semantics are equal to fromChangelogStream(DataStream).

• fromChangelogStream(DataStream, Schema, ChangelogMode): Gives full control about how to interpret a stream as a changelog. The passed ChangelogMode helps the planner to distinguish between insert-only, upsert, or retract behavior.

• toChangelogStream(Table): Reverse operation of fromChangelogStream(DataStream). It produces a stream with instances of org.apache.flink.types.Row and sets the RowKind flag for every record at runtime. All kinds of updating tables are supported by this method. If the input table contains a single rowtime column, it will be propagated into a stream record’s timestamp. Watermarks will be propagated as well.

• toChangelogStream(Table, Schema): Reverse operation of fromChangelogStream(DataStream, Schema). The method can enrich the produced column data types. The planner might insert implicit casts if necessary. It is possible to write out the rowtime as a metadata column.

• toChangelogStream(Table, Schema, ChangelogMode): Gives full control about how to convert a table to a changelog stream. The passed ChangelogMode helps the planner to distinguish between insert-only, upsert, or retract behavior.

From a Table API’s perspective, converting from and to DataStream API is similar to reading from or writing to a virtual table connector that has been defined using a CREATE TABLE DDL in SQL.

Because fromChangelogStream behaves similar to fromDataStream, we recommend reading the previous section before continuing here.

This virtual connector also supports reading and writing the rowtime metadata of the stream record.

The virtual table source implements SupportsSourceWatermark.